Both terrestrial and space solar energy systems are needed which achieve high power and energy to mass ratios, long life, and high efficiency at low cost. For space power systems in particular, low mass is a requirement of increasing importance, especially as the energy levels increase and the energy sub-system mass needed for large satellites and advanced lunar and planetary surface applications becomes a greater fraction of the total system mass. Long life and system operational integrity are necessary to achieve low cost, especially with space solar energy systems. High efficiency, for both thermal energy and electric power needs, is a related requirement to achieve high power to mass ratio and low cost.
Solar concentrators can provide improved energy to mass ratios, long life and high efficiency for both thermal energy and electric power needs, especially if the concentrator reflector elements can be designed to achieve and maintain high optical performance and low mass per unit area.
The present day non-concentrating, flat array solar cell conversion systems are low in efficiency and limited to power levels of the order of 1 to 10 KW.sub.e. In order to achieve higher power levels, the size and number of arrays must be increased, but this increases the complexity of the system integration issues.
The flat arrays are also difficult to protect from the space environment, in particular atomic oxygen. Therefore, the performance of the system decreases with time. This performance degradation requires that more array area be added in order to have the required end-of-life power for the payload.
Within the last few years there have been significant advances in the efficiency of solar cells. However, in order to achieve this efficiency, the sun energy must be concentrated from 100 to 500 times.
A solar photovoltaic concentrator system has the advantage of relatively small cell area located in the central, focal (concentrated irradiance) zone. See U.S. Pat. No. 5,154,777 to J. B. Blackmon, K. W. Stone, R. A. Gerrick, and N. E. Jones.
Concentrators can thus be used to increase the efficiency, reduce mass, reduce the area of obscuration, simplify various system integration issues, and ultimately greatly reduce cost. For these reasons, the use of concentrating solar photovoltaic systems will increase substantially in the future, and the need for a lightweight, high optical performance, long life reflector facet for such concentrators will become a paramount requirement.
Similarly, concentrating solar thermal power systems require low cost, low mass, efficient reflector facets to focus sunlight onto the receivers used to heat the working fluids used in the power conversion units, such as Brayton, Stirling or Rankine engines.
Over the last 5-10 years there have been various advances in the development of lightweight reflective surfaces such as inflatables and the facet for the NASA Space Station Advanced Development Concentrator. The inflatable type of reflective surfaces are delicate, are not expected to achieve long life, and have not achieved the required surface shape and optical quality. The NASA Advanced Development Concentrator type facet has a high reflectance loss such that 10 to 30% of the energy is reflected at angles beyond the receiver due to "print through" reflective surface distortion. There have also been problems with the shape of the facet changing with time and temperature/humidity variations.
One object of the invention is to provide an improved light weight reflector facet.
Another object is the provision of an efficient light weight reflector facet for a solar photovoltaic concentrator system.
A particular object of the invention is to provide a light weight reflector facet of the above type having high reflectivity, low surface "waviness", good structural stability and integrity, good optical quality over temperature extremes, and low mass per unit area.